材料科学技术（英）
Ultrafine particles prepared by evaporating pure Fe in CH4 atmosphere using arc-discharge-heating method. were found to consist of Fe-C solid solution, gamma-Fe and Fe3C phases. Effect of annealing temperature on phase transformation and hyperfine interactions has been investigated by Mossbauer spectroscopy, X-ray diffraction (XRD), differential thermal analysis and thermogravimetry (DTA-TG), transmission electron microscopy (TEM), oxygen determination and vibrating sample magnetometer (VSM) measurements. It was observed that phase transformation of gamma-Fe to alpha-Fe occurs during annealing in vacuum. The mechanism causing the change of hyperfine interactions with annealing temperature differs for Fe-C solution and interstitial compounds. Difference of hyperfine interactions of Fe-C solid solution in the starting sample and its annealed samples is ascribed to the improvement of activation of interstitial carbon atoms. Stress-relieving in structure of annealed Fe3C particle can result in a weak influence on hyperfine interactions. Parameters fitted to the Mossbauer spectra show the existence of superparamagnetism in all the samples. Absorbed and combined oxygen on particle surface of the starting sample were determined.
关键词:
magnetic-properties
Journal of Materials Science
Ultrafine gamma-Fe(C), alpha-Fe(C) and Fe3C particles were prepared by arc-discharge synthesis in a methane atmosphere. The phases, morphology, structure and surface layer of the particles were studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) techniques and X-ray photoelectron spectroscopy (XPS). It was found that the mean particle size ranged from 9.8 to 12.8 nm. The surface of particles mostly consisted of a carbon layer and a little oxide. Phase transformation from gamma-Fe(C) to alpha-Fe(C) was studied by annealing in vacuum and by differential thermal analysis and thermogravimetry (DTA-TGA) measurement. The abundance of gamma-Fe( C) was determined by a magnetization measurement to be approximately 30%. Phase transformation occurred between 300 and 500 degrees C in a flowing argon atmosphere. The Fe3C particles oxidized to alpha-Fe2O3 and carbon dioxide at 610 degrees C or so. (C) 1998 Chapman & Hall.
关键词:
magnetic-properties
Journal of Alloys and Compounds
The structural and magnetic properties of mechanically alloyed samples of Sm12.5Fe87.5 have been studied by adjusting the nitrogenation temperature and time. When the nitrogenation temperature is 500 degrees C, a very short time (less than 5 min) is needed for the nitrogen atoms to be introduced into the lattice. No prominent incubation time is observed, consequently the magnetic properties (including the Curie temperature) of the samples depend only slightly on the nitrogenation time. When nitrogenated at 260 degrees C, two phases, SmFe7 and SmFe7Ndelta, coexist in the samples. The temperature dependences of the magnetic properties of the Sm-Fe-N magnets have been measured up to 600 degrees C. It is found that Sm-Fe-N. powders as well as polymer-bonded magnets made by mechanical alloying have a good thermostability. The coercivity at 200 degrees C of the Sm-Fe-N magnets is greater than 10 kOe. At 550 degrees C the main phase SmFe7Ndelta decomposes into SmN and alpha-Fe and the magnetic properties of the samples are rapidly degraded.
关键词:
magnetic-properties
Applied Physics Letters
Single-phase compounds NdCo(12-x)V(x) (x=2.2-2.6) crystallizing in the tetragonal ThMn(12) structure with space group I4/mmm are synthesized. The lattice parameters a and c increase linearly with increasing V content. The Curie temperature of the compounds shows a linear decrease from 309 K for x=2.2 to 147 K for x=2.6. A domain-wall pinning phenomenon is observed in all of the compounds. It is intriguing that the Nd ions seem to make no contribution to the total moment of the compounds in an applied magnetic field up to 50 kOe. A further increase of the applied field induces a moment of a Nd sublattice that ferromagnetically couples with the moment of a Co sublattice. The instability of the Nd moment in NdCo(12-x)V(x) compounds can be well understood by the local environment effect. (C) 2003 American Institute of Physics.
关键词:
magnetic-properties
Nanostructured Materials
Ultrafine Ni particles have been obtained by evaporating pure Ni in methane gas atmosphere of 100 Torr, and in a mixture of H-2 and Ar of 760 Torr, respectively. The distinct characterization of the layers on two kinds of Ni particle surfaces were investigated by X-ray photoelectron spectroscopy (XPS) and oxygen determination. The morphology and mean particle size were determined by transmission electron spectroscopy (TEM) and subsequent image analysis. The structure of ultrafine Ni particles were identified by X-ray diffraction (XRD). The effect of oxide and carbon layers on magnetization of the particles were studied by vibrating sample magnetometer (VSM). It was observed that a carbon-rich and an oxide of Ni2O3 layers were formed on the surface of Ni particles obtained in methane gas, and in a mixture of H-2 and A(r) gas atmosphere, respectively, it was shown that the methane gas makes resistance to oxidization during evaporating Ni in this gas atmosphere and no nickel carbides or Ni-C solid solution formed. (C) 1998 Acta Metallurgica Inc.
关键词:
magnetic-properties
